This disclosure relates to a methodology for improving the print quality of line-art corners and other fine details as found in both font and image data. Due to the nature of inks, paper and printing, these types of problems are well known in the printing arts.
In the printing arts this problem was typically overcome by manual image customization by hand of any troublesome detail areas. This was particularly the case with fonts or reusable type. For example, ink traps would be added to those areas in a font character where too much ink would otherwise collect and cause smearing. Similarly, detail areas would be sharpened to ensure the desired print result.
This approach of compensating to get a desired result such as sharpening has followed-on from the print arts into the digital imaging arts. As an example, “Digital Typography: An Introduction to Type and Composition for Computer System Design,” by Richard Rubinstein, discusses the desirability of compensating for electrostatic effects which result in toner not being placed on the paper exactly as the bit image specifies. Compensation is depicted there as adding to the image bit map to sharpen convex (outside) corners which would otherwise get rounded over. An alternative compensation is also depicted for handling situations involving concave (inside) corners by removing black printing pixels from the corner region of a shape to make an ink trap. In FIG. 2 there is depicted the bit image data and thereby the desired print image. In FIG. 3 is depicted the rounded-over result when printed on a given printer. FIG. 4 shows the bit image compensation which when printed will yield the desired result as depicted in FIG. 2 above. Note the ears 2 added to the outside corners and the ink trap 4 added to address the inside corner. FIG. 1 also illustrates bit image data/bit image compensation and the resulting printed image for outside corners.
The following patents and patent applications appear to be relevant to manipulation and enhancement of the edges of image shapes. They are incorporated by reference in their entirety for their teaching, particularly of template matching, and of bit map or signal substitution.
In U.S. Pat. No. 4,847,641 to Tung, print enhancement circuitry to enhance the printed image produced by a laser beam printer is interposed between the character generator circuits and the laser drive circuits to modify the laser drive signals provided by the character generator circuits. Bit data representing successive lines of the bit map for a desired image are stored in a first-in first-out (FIFO) buffer. The bit pattern sample window having a central cell (bit) and a selected (arbitrary) number of neighboring bits is compared to a number of matching bit patterns or templates, each of which is associated with an error element or cell. When a logic matching network detects a match, a modification signal associated with a unique compensation cell (bit) is generated. The sample window central bit is then replaced (modified) with the unique compensation bit required by the matching template. In this manner, all bits in a desired bit map, or set of bit maps, are examined and their corresponding laser drive signals modified to compensate for the errors associated with the matched templates in a piece-wise manner.
In U.S. Pat. No. 5,383,036 to Mailloux et al., a method for enhancing the contour fidelity of printed images of two or more colors is described, which includes obtaining a digital representation of the color image and finding color separations of each color. Each color separation is enhanced by a single set of inverse symmetrical templates, the set including templates in which the second template is always the inverse of the first, and the third and fourth templates are 180 degree rotations of the first two. The resulting smoothed color separations are recombined into an enhanced image without separation error.
U.S. Pat. No. 4,437,122 to Walsh et al. discloses methods to enhance the resolution and quality of characters of a system receiving the information initially in the form of video display pixels and providing hard copy output. This is accomplished by storing at least three successive lines of video data in successive, parallel connected shift registers, applying the outputs of the shift registers to a decoder, and generating driving signals for the printer head. The decoder compares the pixels on the same line as well as in preceding and succeeding lines that surround each specific input pixel to generate the printer head driving signals according to whether straight or curved line segments are to be formed. In effect, the printer increases the density of the information elements and simultaneously provides rounding off of character edges and smoothing of diagonals.
U.S. patent application Ser. No. 12/609,475 to Zhang et al. discloses a SUSAN based method to corner sharpening, wherein the method determines whether or not image pixels are associated with a corner as a function of the USAN values generated for the image pixels. Subsequently, one or more pixels associated with a corner are modified to render a corner enhanced image.
There exists a need for techniques which can be easily optimized and provide information on orientation and connectivity. Further, it is desirable to implement corner sharpening techniques utilizing one stage of buffer memory.